feat: Add tier-based vehicle limit enforcement (refs #23)

Backend:
- Add VEHICLE_LIMITS configuration to feature-tiers.ts
- Add getVehicleLimit, canAddVehicle helper functions
- Implement transaction-based limit check with FOR UPDATE locking
- Add VehicleLimitExceededError and 403 TIER_REQUIRED response
- Add countByUserId to VehiclesRepository
- Add comprehensive tests for all limit logic

Frontend:
- Add getResourceLimit, isAtResourceLimit to useTierAccess hook
- Create VehicleLimitDialog component with mobile/desktop modes
- Add useVehicleLimitCheck shared hook for limit state
- Update VehiclesPage with limit checks and lock icon
- Update VehiclesMobileScreen with limit checks
- Add tests for VehicleLimitDialog

Implements vehicle limits per tier (Free: 2, Pro: 5, Enterprise: unlimited)
with race condition prevention and consistent UX across mobile/desktop.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/features/vehicles/tests/unit/vehicles.repository.test.ts

@@ -0,0 +1,66 @@
+/**
+ * @ai-summary Unit tests for VehiclesRepository
+ * @ai-context Tests repository data access methods with mocked database
+ */
+
+import { Pool } from 'pg';
+import { VehiclesRepository } from '../../data/vehicles.repository';
+
+describe('VehiclesRepository', () => {
+  let pool: Pool;
+  let repository: VehiclesRepository;
+
+  beforeEach(() => {
+    pool = {
+      query: jest.fn(),
+    } as any;
+    repository = new VehiclesRepository(pool);
+  });
+
+  describe('countByUserId', () => {
+    it('returns accurate count of active vehicles', async () => {
+      const mockResult = {
+        rows: [{ count: '3' }],
+      };
+      (pool.query as jest.Mock).mockResolvedValue(mockResult);
+
+      const count = await repository.countByUserId('user-123');
+
+      expect(count).toBe(3);
+      expect(pool.query).toHaveBeenCalledWith(
+        expect.stringContaining('SELECT COUNT(*) as count FROM vehicles'),
+        ['user-123']
+      );
+      expect(pool.query).toHaveBeenCalledWith(
+        expect.stringContaining('is_active = true'),
+        ['user-123']
+      );
+    });
+
+    it('returns 0 for user with no vehicles', async () => {
+      const mockResult = {
+        rows: [{ count: '0' }],
+      };
+      (pool.query as jest.Mock).mockResolvedValue(mockResult);
+
+      const count = await repository.countByUserId('user-empty');
+
+      expect(count).toBe(0);
+    });
+
+    it('only counts active vehicles (excludes deleted)', async () => {
+      const mockResult = {
+        rows: [{ count: '2' }],
+      };
+      (pool.query as jest.Mock).mockResolvedValue(mockResult);
+
+      const count = await repository.countByUserId('user-with-deleted');
+
+      expect(count).toBe(2);
+      expect(pool.query).toHaveBeenCalledWith(
+        expect.stringContaining('is_active = true'),
+        ['user-with-deleted']
+      );
+    });
+  });
+});

backend/src/features/vehicles/tests/unit/vehicles.service.test.ts

@@ -53,10 +53,19 @@ describe('VehiclesService', () => {
       findByUserAndVIN: jest.fn(),
       update: jest.fn(),
       softDelete: jest.fn(),
+      countByUserId: jest.fn(),
+    } as any;
+
+    const mockPool = {
+      query: jest.fn(),
+      connect: jest.fn().mockResolvedValue({
+        query: jest.fn(),
+        release: jest.fn(),
+      }),
     } as any;
 
     mockRepository.mockImplementation(() => repositoryInstance);
-    service = new VehiclesService(repositoryInstance);
+    service = new VehiclesService(repositoryInstance, mockPool);
   });
 
   describe('dropdown data integration', () => {